A gas turbine engine as used in an aircraft requires a compressor, a gas turbine and an exhaust duct. The exhaust duct includes discharge nozzles which may be controlled for thrust direction.
All the forces placed on the engine must be transmitted to the airframe. The thrust controlling nozzles place loads similar to, and in the same general location as airframe empennage loads. It is therefore advantageous to integrate the nozzle structural members with the airframe structural members, so as to eliminate the need for redundant structures, with the attendant weights savings. To obtain maximum benefit, these combined loads must be divorced from the loads generated by the gas turbine itself If this is not done, these vectoring and engine loads become statically indeterminate, and introduce bending into the engine, which is detrimental to engine life and performance, negating any weight savings due to integrated structure. Differential expansion, both radially and axially, must be accepted and differential movement, including eccentric movement, must be tolerated between the nozzle structure and the upstream structure. The gas pressure in the order of 3 to 4 atmospheres must also be sealed between the gas turbine exhaust and the nozzle.
Attempts to permit this movement with appropriate sealing using bellows have been a problem because of a structural instability known as squirming, which occurs when a bellows is made sufficiently thin to avoid transferring loads between the nozzle and engine It is desirable to seal such moderate pressures with minimal leakage between the components where relative displacements of up to 4 centimeters in any direction are anticipated.